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The microwave rotational spectrum and the structure of difluorochloromethane McLay, David Boyd

Abstract

The pure rotational spectrum of difluorochloromethane, Freon 22, has been obtained in the K-band microwave region with a Stark modulation spectroscope. Over two hundred spectrum lines, of which the majority are grouped into doublets or quadruplets, have been found. Fifteen doublets and one quadruplet correspond in frequency with previous determinations but the majority of the lines and structures have not previously been recorded. In addition, a definite sestet of lines and two definite triplets have been noted. The grouping into multiplets has been made possible by similarities in the line shape, in the Stark modulation response, and in the effects of chilling the Stark cell with dry ice. The latter effect has been the most important factor in identifying several transitions involving low values of J, the total angular momentum quantum number. The sestet and two quadruplets have been found to be the result of the interaction of the Cl³⁵ quadrupole moment with the electrostatic potential at the site of the nucleus. The sestet represents six of the possible eight lines for a transition from a J=1 rotational level to a J=2 level. The quadruplets represent two transitions involving only J=3 rotational levels. From these three structures have been obtained the complete solution for the spectrum of the molecule containing CI³⁵ and the values of the components of the quadrupole coupling constant for the CI³⁵ nucleus along the principal axes of the angular momentum ellipsoid. In all, eight multiplets have been correctly identified for the molecule containing Cl³⁵ and the two theories of the rotational levels and of the hyperfine structure for an asymmetric top molecule have been verified. From the rotational constants for the molecule containing CI³⁵ the values for the molecule containing CI³⁷ can be closely estimated and the rotational spectrum for the latter case has accordingly been predicted with considerable accuracy. By means of the predicted values, five multiplets and one single line have been identified for the molecule containing CI³⁷ and the theories for the rotational levels and for the hyperfine splittings can be rechecked. From the spectra for the two molecules, five independent rotational constants have been obtained with a precision that compares favourably with the precision of other experimenters in the field of microwave spectroscopy. With these five constants and an approximation to the carbon-hydrogen bond length, the structure of the molecule has been calculated with an accuracy much better than that obtained from electron diffraction measurements. The quadrupole coupling constant for CI³⁵ in Freon 22 has been found to be -71.6 ± 0.5 Mc/s, a value which is about 1 Mc/s larger than the value for the molecule in the solid state. The difference between the components of the quadrupole coupling tensor along two axes perpendicular to the carbon-chlorine bond has been found to be rather small in comparison with the value suggested by bond theory. Some comments are offered on the nature of the chemical bond in the light of these electric quadrupole moments for CI³⁵ and the bond lengths for the carbon-chlorine and carbon-fluorine bonds which have been calculated to be 1.7576 ± .0093 Å and 1.3405 ± .0058 Å respectively for Freon 22.

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